The present invention relates to an air flowmeter, particularly to a hot wire type air flowmeter such as used in the control of a vehicle engine.
In a conventional flowmeter, as described by Japanese Patent Application Laid-Open 184923/1982, the output range of an air flowmeter is set by adjusting resistors connected to the current detecting terminal of a hot wire and the zero point is adjusted by changing the standard input voltage of a differential amplifier.
However it is difficult when using conventional techniques to adjust the output of a flowmeter, since the zero adjustment of the air flowmeter output also changes the output range due to temperature differences in all hot wire products, and so repeated operations for alternatively adjusting the zero point and the output range are needed to fix the zero point and the output range for each target value. That is, conventional techniques have the problem that zero adjustment and output range adjustment can not be independently performed without mutual interference therebetween. Further, conventional techniques can not produce enough compensation to temperature changes, since the output of an air flowmeter can not be compensated over the whole range of possible flow rates, although the output temperature compensation is possible to one point of the flow rates.